1.1 Serial Input and Output

The SI/O pin is an open-drain, bidirectional input/output pin used to serially transfer data to and from the device. The SI/O pin must be pulled high using an external pull‑up resistor and may be wire-ORed with any number of other open‑drain or open‑collector pins from other devices on the same bus. The device also uses the SI/O pin as its voltage source by drawing and storing power during the periods that the pin is pulled high to a voltage level between 1.7V to 3.6V (AT21CS01) and between 2.7V to 4.5V (AT21CS11).